1. Field of the Invention
The present invention relates to a semiconductor acceleration sensor. More specifically, the present invention relates to a MISFET (metal-insulator-semiconductor field-effect transistor) type semiconductor acceleration sensor.
2. Related Arts
In an acceleration sensor for automotive use according to the related art, semiconductor acceleration sensors which can detect low acceleration levels and low frequency levels with good accuracy have been considered well suited to volume production at low cost. Among these there exists an electrostatic capacitance type acceleration sensor disclosed in Japanese Patent Application Laid-open No. 2-134570 and a MISFET type acceleration sensor disclosed in Japanese Patent Application Laid-open No. 4-25764. Both sensors are capable of detecting low acceleration levels and low frequency levels with good accuracy and suited to volume production at low cost. Of these, the latter MISFET type acceleration sensor in particular is considered promising as a sensor having a smaller size. FIGS. 37 and 38 illustrate a MISFET type acceleration sensor disclosed in Japanese Patent Application Laid-open No. 4-25764. Herein, when a gate electrode 93 maintained on an acceleration detection substrate 91 moves up or down in accompaniment to an acceleration, the carrier concentration of a channel region formed within a p-type silicon substrate 92 changes, and fluctuation in an amount of current flowing between a source and a drain is used to detect acceleration. In this drawing, 94 is a cantilever, 95 is a source electrode, 96 is a drain electrode, 97 is a trench, 98, 99, and 100 are wires, and 101 is a package.
However, in a MISFET type acceleration sensor according to FIGS. 37 and 38, the cantilever 94 installed on the gate electrode 93 is maintained on the acceleration detection substrate 91 by means of one beam section. If the beam section is twisted, detection errors increase. That is to say, for an electrostatic capacitance type acceleration sensor, which is structured such that a movable electrode which detects acceleration is installed between two electrodes, and even if the movable electrode is twisted, the change thereof is canceled out and no problem exists. However, if twisting of the movable electrode occurs in a MISFET type acceleration sensor, detection error and faulty operation are caused.